The present invention relates to an oil recovery system for a closed type centrifugal refrigerating machine which is capable of effectively recovering lubricating oil that has leaked in a refrigerant even when the refrigeration load is small.
The refrigerating cycle of the closed type centrifugal refrigerating machine will be explained below with reference to FIG. 3.
Refrigerant gas A that evaporates in an evaporator 1 is sucked into a compressor 2. The volumetric flow of refrigerant gas sucked into the compressor 2 is controlled by a suction volume control valve 3, for example, a suction vane, in accordance with the magnitude of refrigeration load. The refrigerant gas compressed in the compressor 2 is discharged to a condenser 4 where it is cooled to condense into its liquid phase. The now liquid refrigerant returns to the evaporator 1 through an expansion mechanism 5, for example, an orifice. In this system, a bearing for an impeller 6 in the compressor 2, a speed increasing gear 7, etc. are lubricated by a lubricating system (not shown), which is separate from the refrigerant system.
In the closed type centrifugal refrigerating machine, the refrigerant system and the lubricating system are shut off from the outside as a whole, and these two systems are separated from each other by a sealing device. However, there is unavoidable communication between the two systems, although it is only a little. In general, oil leaks into the refrigerant system from a shaft extending portion of the compressor 2, as shown by B in FIG. 3. The leakage oil reaches the evaporator 1 through the condenser 4, together with the refrigerant gas. Since the refrigerant alone evaporates in the evaporator 1, the oil remains therein.
Usually, however, refrigerant mist is sucked into the compressor 2, together with the refrigerant gas that evaporates in the evaporator 1, so that the leakage oil is eventually dissolved in the mist and carried out of the evaporator 1. An oil sump 8 is provided at the suction side of the compressor 2, as shown in FIGS. 4 and 5. Oil that gathers in the oil sump 8 is sucked and recovered to an oil tank 13 by an ejector 11 through an ejector filter 12. The ejector 11 is disposed in the intermediate portion of a piping that connects together a scroll portion of a discharge gas passage for the gas compressed in the compressor 2 and the oil tank 13 that is equalized in pressure with the upstream side of the suction volume control valve 3. The ejector 11 uses a pressure difference between the discharge pressure and the cooler pressure as driving force to return the oil in the oil sump 8 to the oil tank 13 through the ejector filter 12 by the ejector effect caused by the refrigerant gas.
In FIG. 3, reference numeral 24 denotes a main motor for the compressor 2, and 9 a bypass line that bypasses part of the refrigerant gas flowing into the condenser 4 from the compressor 2 to the evaporator 1, the bypass line 9 being provided with a hot gas bypass valve 10. Reference numerals 14 and 15 denote a chilled water inlet and a chilled water outlet of the evaporator 1, and 16 and 17 a cooling water inlet and a cooling water outlet of the condenser 4. In addition, reference numeral 18 denotes an oil cooler, 19 a purge condenser, 20 a refrigerant pump, 21 a strainer, and 22 a refrigerant cooler.
When the conventional closed type centrifugal refrigerating machine runs in a full-load condition or a load condition which is close to it, the flow velocity of the refrigerant gas is sufficiently high that an adequate amount of refrigerant mist can accompany the refrigerant gas to effect satisfactory recovery of the leakage oil.
However, when the machine is in a partial-load condition, the amount of refrigerant mist accompanying the refrigerant gas is small, so that the amount of oil recovered becomes smaller than the amount of leakage oil, resulting in an increase in the amount of oil remaining in the evaporator 1, being dissolved in the refrigerant. In consequence, the amount of oil in the lubricating system decreases, and a low oil pressure tripping device is eventually activated to stop the refrigerating machine.
Thus, it has heretofore been necessary to supply oil to the lubricating system in order to continue the operation of the refrigerating machine without interruption.
When the load increases to a level at which a full-load running is available, the oil recovery function by the accompaniment of refrigerant gas is restored. As a result, the oil level in the oil tank rises, so that an excess of oil must be taken out of the oil tank, which burdens the operator with a very troublesome task.
There is another problem that, when the concentration of oil dissolved in the liquid refrigerant increases, the contamination of the tube in the evaporator 1 is promoted to check the heat transfer.
To prevent the occurrence of the above-described problem, one type of centrifugal refrigerating machine makes use of a differential pressure which is produced across the suction volume control valve 3 that is automatically closed during a partial-load running, to lead the liquid refrigerant having the oil dissolved therein from the evaporator 1 to the downstream side of the suction volume control valve 3, thereby recovering the leakage oil. More specifically, when the refrigerating machine is in a partial-load condition, the opening of the suction volume control valve 3 is small, so that the differential pressure across the control valve 3 becomes sufficiently large to enable the liquid refrigerant having the leakage oil dissolved therein to be sucked into the oil sump 8 by virtue of a difference between the cooler pressure and the pressure at the downstream side of the control valve 3, thus recovering the leakage oil. However, even in a centrifugal refrigerating machine with such an oil recovery function, if the compressor 2 turns on/off under a small refrigeration load condition, the temperature of chilled water rises when the compressor 2 is off, and when it turns on, the suction volume control valve 3 in the prior art is fully opened because the chilled water temperature is relatively high for the moment. However, since the load is small, the chilled water temperature lowers within a short time, thus causing the compressor 2 to turn off. Accordingly, if the compressor 2 starts and stops frequently, it is impossible to obtain sufficient time to remove the liquid refrigerant having the leakage oil dissolved therein by making use of a differential pressure across the suction volume control valve 3.
In general, the oil pump in the lubricating system performs the residual running for a predetermined period of time after suspension of the compressor 2, and the leakage of oil into the refrigerant system occurs even during the suspension of the compressor 2. For this reason, it has heretofore been impossible to solve completely the problem that the leakage oil remains in the evaporator 1 and cannot be recovered to the oil tank.